Acceleration measuring system



United States Patent ACCELERATION MEASURING SYSTEM Raymond W.Ketchledge, Middlesex, N. J., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York Originalapplication November 14, 1947, Serial No. 785,928, now Patent No.2,650,991, dated September 1, 1953. Divided and this application May 19,1950, Serial No. 162,920

Claims. (Cl. 264-1) This application is a division of application,Serial No. 785,928, filed November 14, 1947, now Patent No. 2,650,- 991,which discloses an improved accelerometer, one form of which may beemployed to measure shock or acceleration along a specified directionand is thus designated a line-responsive accelerometer.

' This invention relates to an improved system of apparatus forcontinuously determining the component in a given direction of thevelocity attained by an object subjected to acceleration.

It is the object of this invention to provide a novel system ofapparatus in which a line-responsive accelerometer may be employed tomeasure the component in a given direction of an acceleration.

A feature of the invention resides in the provision of apparatusincluding means for rotating a line-responsive accelerometer about anaxis normal to a given direction at a desired frequency of rotation toobtain an alternating voltage of amplitude proportional to the componentof acceleration parallel to the plane of rotation, of frequency equal tothe frequency of rotation and of phase correspondent to a radialdirection in the plane of rotation, in combination with means forcomparing this voltage with a reference alternating voltage of constantamplitube, of frequency equal to the frequency of rotation of theaccelerometer and of phase correspondent to a selected direction in theplane of rotation.

The invention will be understood from the following description withreference to the accompanying drawing in which:

, Fig. 1 is a diagram of a line-responsive accelerometer;

Fig. 2 shows a line-responsive accelerometer distinguishing left fromright shocks;

Fig. 3 diagrammatically represents an apparatus employing aline-responsive accelerometer;

Fig. 4 is a schematic of a phase-sensitive rectifier empolyed to comparethe accelerometer voltage with a reference voltage of the same frequencybut of fixed determinable phase;

Figs. 5, 6 and 7 are curves showing the relationships of the phase ofthe accelerometer voltage with respect to the reference voltage, thephase of the reference voltage in each case corresponding to a selecteddirection in the plane of rotation of the accelerometer.

Referring to Fig. 1, numeral 10 designates generally an array ofpiezoelectric crystals, described in greater detail in the parentapplication, by which a change in liquid pressure resulting from anacceleration is translated into an electrical voltage related to thatchange. To the array 10 are glued, one at each end thereof, the ceramicdiscs 11, 12. These, and other elements of the assembly, are housed in acylindrical core made up of the two sleeves 13 and 14, sleeve 13 beingexternally threaded to be secured into internally threaded sleeve 14. Aninternally threaded clamping ring preserves the desired lengthwiserelationship of sleeves 13 and 14.

2,726,074 Fatented Dec. 6, 1955 The internal surfaces of sleeves 13 and14 are prolongations of each other, and smoothly fitting within them arethe discs 11 and 12. The end portions of sleeves 13 and 14 are eachinternally cut out to leavea shoulder which is adjusted to be nearlyflush with the outer face of the corresponding ceramic disc. Theendportions so cut out are internally threaded so that neoprene gaskets16 and 17, surrounding discs 11 and 12, respectively, may be assembledwith their outer surfaces flush with the like faces of the discs, andheld firmly so by clamping rings 18, 19. Between gasket 16 and clampingring 18 is tightly held in a diaphragm 20 of stainless steel or rubberwhich is in firmcontact with the outer face of disc 11; diaphragm 21 issimilarly held between gasket 17 and ring 19 and in firm contact withthe outer face of disc 12.

Leads 22, 22' are brought from crystal array 10 through holes ininsulating plugs 23,23 threaded into radial pole sleeve 14, thence viaany desired path (for simplicity, notches) through sleeve 14 and plug139 for connection to a measuring circuit.

While a piezoelectric crystal is preferred as the pressure-responsiveelement, other such elements may serve in the present invention. Forexample, use may be made of the magnetostriction of nickel, apermanently magnetized nickel tube may be held under slight initiallengthwise compression between discs 11 and 12 and surrounded by a'coilterminating in leads 22, 22. When the compression of the rod changes, acorresponding voltage is induced in the coil. Other pressure-sensitiveelements will occur to those acquainted with the art.

The theory of operation of the instrument is based on the familiar factthat at the bottom of a column of liquid of height H and density P thehydrostatic pressure is PgH, Wh6le g is the acceleration of gravity. Ifsuch a column is subjected to any other acceleration G, the pressurebecomes PGH, and this effect is produced in a horizontal column oflength H exposed to a horizontal acceleration. In each case, thedimension in the direction of the acceleration determines the resultingpressure at the near end of the column in that direction; the bottomwhen the column is of height H and the acceleration overcomes gravity;the left end of a horizontal column of length H when the acceleration isdirected to the right in a horizontal line. Y

In the line-responsive accelerometer 45, plugs 138 and 139 arehemispheres (or cylinders) seating snugly against diaphragms 20 and 21,respectively, Plugs 138 and 139 are solid except for lengthwisecylindrical bores of small diameter, C51 and C52, extending belowclosing screws 30 and 31 to the diaphragm surface. Cavities C51 and C52are filled with mercury. Such an accelerometer responds to accelerationsin the line XY without regard to whether shocks are from X toward Y orreversely. To make accelerometer 45 capable of distinguishing right fromleft shocks, C51 (say) is left empty, diaphragm 20 is made rigid and aninitial pressure is created in cavity C52. A more sensitive device ofthis character is shown in Fig. 2.

The same principle, with appropriate departure in design, is applied inthe discriminating line-responsive instrument diagrammed in Fig. 2, andgenerally designated by numeral 50.

Referring now to Fig. 2, the general form of the enclosing case is thesame as that of the instrument shown in Fig. 1. In Fig. 2, however, apair of crystal arrays 10A and 10B mounted internally and lengthwise ofthe sleeves 13 and 14, are separated by a lengthwise column of mercury,filling cavity C53 in steel cylinder 56 which fits snugly within sleeves13 and 14. Cylinder 56 is provided with a threaded filling hole 60,closed after filling with plug 61, and is inserted through hole 62drilled radially through sleeve 14. Crystals A and 10B are similar inconstruction to crystal 10 of Fig. 1.

Cylinder 56 is turned internally at each end to receive the annulargaskets 63, 64. After assembly of the instrument, the end surfaces ofcylinder 56 are flush with the outer surfaces of gaskets 63 and 64, andagainst these surfaces are provided rubber diaphragms 65 and 66, incontact as in the accelerometer of Fig. 1 with ceramic discs 69 and 70,respectively, through which the hydrostatic pressure in the liquidfilling cavity C53 is effective on the respectively adjacent ends ofcrystals 10A and 10B to which the ceramic discs are glued as beforedescribed. At their ends remote from the discs, the crystals are gluedto discs 71, 72, say of plastic material, beyond which are end plugs 28and 29. Plugs 28 and 29 are centrally tapped to receive screws 30 and31, respectively, by means of which the crystals are given any desiredinitial compression after assembly of the instrument.

Leads 73, 74 for crystal 10A are taken through insulating bushings 75,76 in sleeve 13; for crystal 10B, leads 77, 78 pass through similarbushings 79, 80. The assembly of the instrument is obvious and is hereunnecessary to describe.

The crystals have been given an initial compression by means of screws30, 31 and lead 73 is connected to lead 77, while leads 74 and 78 areconnected each to one terminal of zero center meter 27. The chargedeveloped by the initial compression rapidly leaks away through meter27.

Let the crystals be each so poled that further lengthwise compressionmakes leads 74 and 77 positive and leads 73 and 78 negative. Reductionin lengthwise compression reverses these polarities in each case. Let itbe assumed that a shock, in the direction of the arrow in Fig. 2, isapplied to the accelerometer. The result will be an increasedhydrostatic pressure at the left end of cavity C53, a decrease in thepressure at the right end thereof. The piezoelectric effect then, withleads 73 and 77 connected as shown, is to develop two voltages in seriesacross meter 27. Lead 74, connected to the upper terminal of meter 27',is positive; lead 78, connected to the lower terminal of the meter, isnegative.

Obviously, if the shock is oppositely directed, the voltage across meter27 is reversed. The meter deflection is therefore in one direction for ashock from left to right, in the opposite direction for a right-to-leftshock.

The sensitivity of the accelerometer is proportional to the depth of themercury pool in the direction of the acceleration. With the design ofFigs. 1 and 2, the pressure to which the crystal responds is that at thenear end of the mercury column rather than the average pressure from endto end. Thus, with a 2.5-inch length for cavity C53 of Fig. 2, about 5volts/ g would be obtained. One-millionth g, or one-thousandth of acentimeter per second squared, would provide 5 microvolts, a quantityreadily amplified for convenient measurement. The application of theinvention to the accurate measurement of gravity is possible on land, atsea or in the air, whereever a stable platform is available; for sea andair work, gyroscopic stabilization is disclosed, for example, in Patents2,014,825, September 17, 1935, to I. P. Watson, and 1,840,104, January5, 1932, to H. Anschiitz-Kaempfe.

Referring to Fig. 3, a system of apparatus for the measurement ofgravity is schematically shown. Motof 85, supported on stable platform86, is supplied with power from a source symbolized by battery 87 androtates shaft 88 in bearings supported parallel to platform 86 bypillars 91 and 92. Between pillars 91 and 92, shaft 88 carries drum 89which supports, at right angles to shaft 83, accelerometer 50 of Fig. 2and counterweight 9S. Slip rings 94 and 98, insulated from each otherand from shaft 88, are mounted on the shaft at any convenient positionand to them are connected, respectively, leads 74 and 78 fromaccelerometer 50.

It will be readily understood that the centrifugal force on theaccelerometer is constant and gives rise to a steady voltage betweenleads 74, 78 which rapidly vanishes. However, downward acceleration ofgravity in one vertical position of accelerometer 50 is productive of anadditional piezoelectric voltage which reverses sign when theaccelerometer reaches the opposite vertical position. If p is thedensity of mercury, l the length of the mercury-filled cavity C53 and fthe angular speed of shaft 88 in revolutions per second, there appearsbetween leads 74, 78 an alternating voltage proportional to gl sin 21ft,of phase dependent on the sense of rotation of shaft 88.

Making contact with slip rings 94, 98 are brushes 96, 99 respectively,which are connected to electrical filter 100 which is preferably tunedto the rotation frequency to include extraneous signals, providing atits output only pgl sin 21rft, the generated alternating voltage. Thisoutput voltage, amplified by amplifier 101, is by switch S appliedeither to meter 102 or by leads 90 to rectifier 103.

The alternating voltage between leads 74, 78 is proportional inamplitude to the acceleration to be measured, but is of phase dependentupon the radial direction of the acceleration; the amplitude is the samewhatever this radial direction, so the apparatus of Fig. 3 so fardescribed is plane-responsive. To confine its response to a prescribeddirection in the plane of rotation of accelerometer 50, it is necessaryto compare the output voltage from amplifier 101 with a referencevoltage of the same frequency but of fixed determinable phase.

Rectifier 103 must therefore by a phase-sensitive device, an example ofsuch being the circuit disclosed in my United States Patent 2,434,273granted January 13, 1948. The accelerometer voltage between leads 90 iscompared in rectifier 103 with a reference voltage of the same frequencyderived from generator 107 driven by shaft 88 and impressed throughphase shifter 108 on rectifier 103 via leads 110. Generator 107 is ofconventional design, and generates a voltage of fixed phase and ofamplitude large compared with the accelerometer voltage. Phase shifter108 is preferably of the type disclosed by H. Nyquist in Patent1,717,400, granted June 18, 1929; it permits rotation, through anydesired angle, of the phase of the voltage from generator 107. Both mypatent above referred to and the patent to Nyquist are assigned to thesame assignee as the present invention.

The operation of rectifier 103 will be more clearly understood byreference to Figs. 4, 5, 6 and 7. The signal voltage is a sine wavewhose magnitude and phase are due respectively to the magnitude anddirection of the acceleration. The function of the phase sensitiverectifier is to determine the component of acceleration in a particulardirection, that direction being determined by the phase of the referencevoltage.

As shown on Fig. 4, the signal voltage on leads 90 is applied totransformer which has a center tapped secondary. Thus the voltage fromlead 124 to lead is equal and opposite to that from lead 124 to lead126. Likewise, the reference voltage on leads 110 is applied to asimilar transformer 121 whose center tapped secondary is used to supplyvoltage to diodes 122 and 123. These diodes may be either vacuum tube orsemiconductor diodes. The reference wave is made large compared to thesignal wave so that the conduction or non-conduction of the diodes 122,123 is controlled principally by the reference wave. Thus for oneinstantaneous polarity of the reference wave, say lead 127 positive tolead 128, diodes 123 will conduct and diodes 122 will be biased tonon-conduction. This action connects lead 126 to leads 127 and 128 viadiodes 123. Lead 129 is a center tap on the secondary On transformer 121and thus the impedance of lead 129 to the combination of leads 127 and128 is low. This is due to the cane elation of voltage drops in the twohalves in the secondary of transformer 121. Thus, for this polarity ofthe reference wave, output lead 129 is effectively connected to lead126.

On the opposite half-cycle of the reference Wave lead 128 is positivewith respect to lead 127. Diodes 122 conduct to thereby connect lead 129to lead 125. It can be seen that the action of diodes 122, 123 is likethat of a single pole, double-throw switch in which lead 129 isalternately connected to leads 125 or 126 under the control of thereference wave polarity.

Figs. 5, 6 and 7 show three conditions of signal wave phase with respectto the reference wave phase. In Fig. the signal is in phase with thereference wave and the switching action serves only to invert thepolarity of the negative excursions of the signal wave. The output waveis, therefore, like that of a conventional full-wave rectifier andcontains a direct-current component proportional to the amplitude of thesignal Wave.

In Fig. 6 the signal wave is reversed in polarity or has a 180-degreephase with respect to the reference wave. Here it is the positive cyclesof the signal that occur at the time the switching action inverts thepolarity. Consequenly, the output signal is the same as before exceptthat the output polarity is negative.

In Fig. 7 a 90-degree phase difference between the signal and referencewaves is shown. Here the switching action occurs at the signal peaksrather than the signal zeros and the output wave has a zerodirect-current component. It'can readily be shown that thedirect-current component of the output wave is proportional to themagnitude of the signal wave times the cosine of the phase differencebetween the signal and reference waves. Thus, if the signal wavemagnitude represents magnitude of acceleraiton and if its phase anglerepresents direction of acceleration, then the direct-current output ofphase sensitive rectifier 103 represents the vector component ofacceleration in the direction represented by the phase of the referencewave.

Phase shifter 108 may be adjusted to make the phase of the voltage onleads 110 correspond with any desired radius in the plane of rotation ofaccelerometer 50. There results at the output of rectifier 103 aunidirectional voltage proportional exclusively to the accelerationcomponent along the selected radius and of polarity corresponding to thesense of the acceleration in that direction.

The adjustment of phase shifter 108 thus makes the system of Fig. 3line-responsive in any desired direction lying in the planeperpendicular to the axis of shaft 83.

When gravity is the quantity to be measured, its direction is knownbeforehand to be downward and the phase of the alternating voltage isimmaterial. Therefore, meter 102 may be a conventional alternatingcurrent voltmeter.

It should be recognized that platform 86, shown horizontal in Fig. 3,may be vertical as well. If shaft 88 is mounted to rotate in a verticalposition, and the sense of rotation is fixed, the apparatus of Fig. 3 isadapted to measure the horizontal acceleration of a vessel carrying it.For this purpose, switch S is thrown to supply the output voltage ofamplifier 101 to rectifier 103. The output voltage of rectifier 103 maybe read, if desired, on direct current voltmeter 104; it is positive,say, for acceleration forward, negative for the reverse acceleration.Obviously, this rectified output voltage may be applied to anintegrating circuit 105. Circuit 105 may, for example, be an RC circuitsuch as is shown in Patent 2,099,536, November 16, 1937, to S. A.Scherbatskoy et al.; it delivers to recorder 106 a voltage proportionalto the time integral of the voltage output from accelerometer 50, and soproportional to the time integral of the acceleration, i. e., to thehorizontal velocity of the vessel.

The combination of a plurality of such apparatus as shown in Fig. 3,with shafts rotating about mutually perpendicular axes, of coursepermits the determination of the corresponding velocity components.Accelerometer 45 of Fig. 1, modified as described for left-rightdiscrimination, may replace accelerometer 50 in the system of Fig. 3.

in accelerometers herein described, an initial hydrostatic pressure inthe mercury produces a momentary voltage which rapidly vanishes if theinstrument is undisturbed. Thus the voltage appearing in response to anacceleration is a function of the increment of hydrostatic pressureproduced by the acceleration.

What is claimed is:

1. Apparatus for measuring the component in a given direction of anacceleration comprising a line-responsive accelerometer including anelectrical generator developing a voltage responsively to anacceleration, means for rotating the accelerometer at a desiredfrequency of rotation about an axis normal to the given direction, saidrotating means including means for supporting the accelerometer remotelyfrom the axis with its line of response normal thereto whereby thegenerator develops an alternating voltage of amplitude proportional tothe component of the acceleration parallel to the plane of rotation, ofphase correspondent to the radial direction in that plane of saidcomponent and of frequency equal to the frequency of rotation, meanscontrolled by the rotating means for generating a reference alternatingvoltage of constant amplitude and phase and of frequency equal to thefrequency of rotation of the accelerometer, phaseshifting means forderiving from the reference voltage an alternating voltage of phasecorrespondent to the given direction, a phase sensitive rectifier forcomparing the last-named voltage with the voltage developed by theaccelerometer, thereby deriving a unidirectional voltage proportional tothe component of the acceleration in a given direction, and means formeasuring the unidirectional voltage.

2. A system of apparatus for continuously determining the component in agiven direction of the velocity attained by an object subjected toacceleration comprising apparatus as in claim 1 for deriving aunidirectional voltage continuously proportional to the component of theacceleration in the given direction, means for integrating with respectto time said voltage and means for continuously recording the magnitudeof the integrated voltage.

3. A device of the class described for the detection and measurement ofexternal forces, comprising a casing, at least one inertia-type sensingcouple in said casing and sensitive to accelerations in a predeterminedgeometrical direction with respect thereto, means mounting said couplein said casing for rotation about an axis different from said direction,power means for rotating said sensing couple at a predetermined speedabout said axis, and means for continuously measuring the output of saidcouple during its rotation.

4. The device according to claim 3, wherein the sensing couple includesan inertia element mounted between a pair of piezoelectric crystals.

5. The device according to claim 3, wherein the sensing couple includesan inertia element mounted between a pair of magnetostrictive elements.

References Cited in the file of this patent UNITED STATES PATENTS636,844 Reed Nov. 14, 1899 2,319,940 Manison May 25, 1943 2,359,158Rushing et al Sept. 26, 1944 2,371,626 Kecskemeti Mar. 20, 19452,513,340 Lyman July 4, 1950 2,554,512 Varian May 29, 1951 2,648,055Smith Aug. 4, 1953 FOREIGN PATENTS 719,762 France Feb. 10, 1932

